The proposal is a correlative study of ultrastructural and electrophysiological events during sperm-egg (Lytechinus) interaction which centers on identifying the primary events involved with egg activation. Sperm attachment initiates one or more signals from the egg surface which results in activation. One possibility is that an excitatory process is initiated immediately after sperm attachment as a result of a sperm ligand-egg receptor type of interaction. Alternatively, the excitatory phase may be induced by fusion between the sperm and egg plasma membranes. It is not known when the gamete plasma membrane fuse, nor is it known whether fusion of the sperm and egg plasma membranes is necessary to induce egg activation. These gaps in our knowledge are addressed by this proposal. The specific aims of this proposal include: (1) to establish by electron microscopic observations how the interacting sperm and eggs are structurally associated at different intervals at and following the onset of the activation current; (2) the ultrastructural relation of interacting gamete plasma membrane in voltage-clamped eggs with changes in the activation current; (3) to correlate the ultrastructural relation of interacting gamete plasma membranes in voltage-clamped oocytes; and (4) to correlate the ultrastructural relation of the plasma membranes of interacting gametes in which the egg has been treated with an agent (cytochalasin B) that blocks sperm incorporation but not egg activation with changes in the activation current. These proposed experiments will establish when fusion of the gamete plasma membranes occurs relative to the onset of the activation current and whether fusion of the sperm and egg plasma membranes is requisite for egg activation.